The efficiency of ablative photodecomposition of poly(methyl methacrylate) caused by pulsed radiation at 193 nm from an excimer laser (argon–fluorine fill) has been studied as a function of fluence. A threshold for the onset of the process lies at 10 mJ/cm2per pulse of 14 ns. The efficiency of the process increases rapidly with fluence up to 250 mJ/cm2per pulse after which it levels off sharply. The etch depth of ∼3100 Å/pulse at 250 mJ/cm2can be compared to a depth of 4800 Å/pulse at a fluence of>20 J/cm2/pulse. A model is suggested for the ablative photodecomposition of organic materials and the photochemistry of the process at different fluences is discussed. The variation in the efficiency with the composition of the polymer is briefly considered.